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What are the essentials for a cruise holiday? Sun hat? Kindle? Condoms?

Given that the average age of a cruise passenger is 57, you might be surprised by the third item on that list. Indeed, a recent survey by Cruise.co.uk found that very few cruise lines bother to cater for the sexually active on board their ships.

Of the biggest cruise lines surveyed by the company at the end of last year, only six (Royal Caribbean, Norwegian Cruise Line, MCS Cruises, Holland America, Disney and Crystal Cruises) stock condoms, and just three (MSC Cruises, P&O Cruises and Cunard) offer the morning after pill. (Oddly, while P&O, Cunard and Princess Cruises all sell the morning after pill, none sells condoms).

Given that a report by the Telegraph last September found that the number of men and women over the age of 45 who were diagnosed with an STI doubled between 2000 and 2009, this is a pretty poor show.

Then there's the recent rise in cruises for swingers, naturists and even "cougars" (older women in search of young bucks) to consider. In November American-based swingersCruise.com chartered a 3,634-passenger Royal Caribbean ship for a "lifestyle" cruise. "There will be no open sexual activity allowed anywhere except your cabin and dedicated play rooms," visitors to the website were reassured.

Furthermore, increasing numbers of cruise lines are catering for solo travellers with more affordable, single cabins, and dedicated areas for singles to mix and mingle. And mingle they do. Another Cruise.co.uk survey recently suggested that one in five passengers have cheated on their partner during a cruise (see full details below).

Dr Richard Dawood, Telegraph Travel's travel health expert, said the failure to cater to amorous cruise passengers come as no surprise. "Medical facilities on board today’s cruise ships are highly developed, but contraception and STI prevention among passengers are a long way from being the main focus."

Unless, perhaps, you are travelling with an Italian line. MSC Cruises trumped all those surveyed, ticking boxes for condoms, pregnancy tests and even lubrication. The morning after pill is available on prescription.

Giles Hawke, MSC Cruises executive director UK & Ireland, said: "We are committed to ensuring that all of our guests have a safe and worry-free holiday, and part of that means having on-board shops stocked for all eventualities."

Cruise lines are increasingly catering to the singles market

Sexual health charity FPA said its research showed that many older people were meeting new partners through dating websites and holidays but appeared to be “oblivious” about the need for safe sex.

Cruise.co.uk cites a report from Government body NaTHNaC (The National Travel Health Network and Centre) about the rise of STIs in the over 45s. Along with tips about safe sunbathing and vaccinations comes the advice: "Remember alcohol can lower your inhibitions, potentially putting you at risk of sexually transmitted infections. If there is a chance you might have casual sex, be prepared: take in-date, good quality condoms with you."

"The message for solo travellers is very clear," says Dr Dawood. "If there is even the remotest prospect of a romance on board, pack condoms and take careful precautions against unwanted pregnancy."

With expedition cruising on the increase and lines such as Seabourn attracting an increasingly younger clientele the rules don't just apply to solo travellers.

“We attract guests of all ages and want them to have a great time while they are with us," said Mr Hawke. "Away from the stress of everyday life it’s easier for couples to make time for one another – whether that’s in our spa, one of our restaurants or bars, the nightclub… or their cabin.

“All of our ships stock a selection of condoms and lubes, and the morning after pill is available when prescribed by the ship’s doctor.”

Shockingly, one in five passengers confessed to cheating on their partner on a cruise ship, with a further 80 per cent of these stating their partner was on board at the time, according to the Cruise.co.uk survey.

The website posted the "completely anonymous" adults-only poll on its Facebook page to discover whether the "liberated" feeling people have on a cruise holiday might be a contributing factor in the rise of STIs reported by NaTHNaC.

“Following Government recommendations towards the end of last year for older generations to take condoms on holiday we decided to survey our database of over 200,000 cruise enthusiasts on their attitudes towards sexual health," said Seamus Conlon, managing director of Cruise.co.uk.

"Clearly the Government’s recommendation to iterate the importance of cruisers protecting themselves against STIs is a very important one."

Comments on the page from customers ranged from: "I'm afraid to open link," and "Is this a wind up?" to "I had one foot on the quayside at all times" and "What sad git comments on this?"

The survey, conducted via surveymonkey.com in December, comes with a warning: "Not for the faint of heart." To date fewer than 400 people have responded. Of those who did, a third admitted they engage in regular sexual activities while on board and some said they would not leave home without their sex toys.

"We'd normally expect at least 1,000 responses to our survey requests," said a spokesperson for the company. "People are being a bit shy about coming forward with opinions."